The present invention relates to a method and device for measuring at least one parameter of a metal bed. By the term xe2x80x9cmetal bedxe2x80x9d is meant metal in solid as well as liquid form. The term comprises arbitrary shapes of the bed and, thus, it is not limited to comprise an elongated web.
In the industry, essentially the metallurgical industry, there is a great need for devices for measuring parameters, such as speed, of a metal bed in motion, for example, when controlling the metal flow in continuous casting or the stirring speed in a chill mould.
A device for measuring the speed of a molten metal is known from U.S. Pat. No. 4,144,756. According to this publication, the device comprises one coil which generates a magnetic field and two coils which have a receiving function. When the molten metal flows past the magnetic field producing coil, the charges in the molten metal sense a lorentz force, which gives rise to an eddy current that generates a second, or induced, magnetic field. The other coils receive the induced magnetic field and give rise to a signal via a circuitry, the signal being proportional to the speed of the molten metal. A drawback of the known device is, however, its fairly complicated design which has many components and that it is not reliable enough as regards ambient, disturbing fields, if any, which makes the device difficult to use. Another drawback is that the direction of motion of the metal flow cannot be determined by the device without the aid of other devices.
JP 57-199917 is included in Patent Abstracts of Japan and discloses an electromagnetic fluxmeter for molten metal. The fluxmeter comprises an annular, wound coil which is arranged circumferentially on a tube, in which metal flows. A magnetic field is generated by the annular, circumferential coil, whereby lines of flux parallel to the metal bed are formed. The magnetic field gives rise to eddy currents in the molten metal. A force which is related to the eddy currents affects the coil in the downstream direction of the metal flow. The circumferential coil on the tube is supplied with four load sensors which are relatively offset at an angle of 90xc2x0. The load sensors are used to measure the force which affects the circumferential coil, the force being proportional to the flow rate. A drawback of this prior-art construction is that it is very limited as regards possible fields of application, it is bulky and lacks the desired flexibility.
An object of the invention is to eliminate the problems of prior-art techniques by providing a method for measuring at least one parameter, such as the speed or the direction of motion, of a metal bed, the method being simple and reliable.
Another object of the invention is to provide a device for measuring at least one parameter of the metal bed, the device being simple, small and easy to handle.
Yet another object of the invention is to provide a device which is durable and which is easy to adapt to various applications and environmental conditions thereof.
The above-mentioned objects are achieved according to the invention by a method and a device which have the features defined in the accompanying claims.
According to the present invention, the problem of measuring a parameter of a metal bed is tackled in a new way by using a technique which, unlike the known electromagnetic detection, uses xe2x80x9cmechanicalxe2x80x9d detection instead.
The invention is based on the knowledge that great flexibility and liberty of placing can be achieved by measuring from only one side of the metal bed. Nor is one bound to special dimensions of the metal bed but, on the contrary, the invention may advantageously be used for measuring on many different types of metal beds.
According to a first aspect of the present invention, a method is thus provided for contactless measuring of at least one parameter of a metal bed, the method comprising the steps of:
generating a magnetic field so that eddy currents are generated in the metal bed as the metal bed moves relative to the magnetic field, which is generated from only one side of the metal bed;
making the metal bed and the magnetic field move relative to one another;
adapting a means in connection with the metal bed so that it is subjected to the action of the force which is related to the eddy currents;
detecting the action of the force, which is induced by the eddy currents, on said means; and
determining the parameter of the metal bed on the basis of the detected force which is a function of the parameter.
According to another aspect of the present invention, a device is provided for contactless measuring of at least one parameter of a metal bed, the device comprising
first means for generating a magnetic field so that eddy currents are generated in the metal bed as the metal bed and said first means move relative to each other, said first means being adapted to generate the magnetic field from only one side of the metal bed;
second means which interact with the eddy currents and are arranged so that they are affected by a force which is related to the eddy currents; and
third means which are adapted to detect the action of the force on said second means, the detected force being a function of the desired parameter of the metal bed.
The device thus has a non-circumferential configuration and is designed to be placed on an optional side of the metal bed. Naturally, this gives a great possibility of selecting a measuring point and also of varying the distance to the metal bed.
The desired parameter may be any parameter that can affect the generated eddy currents and, thus, the force. Some examples of parameters are the speed of the metal bed as regards size or direction, the distance to the surface of the metal bed, the thickness or conductivity of the metal bed, or a combination thereof. Thanks to the device being placed on only one side of the metal bed, and preferably with such a suspension that its general extension is transversely of, preferably substantially perpendicular to, the metal bed, motions in all directions in the plane of the metal bed can be detected.
The device for measuring a parameter of a metal bed according to the present invention comprises first means for generating a magnetic field so that parameter-dependent eddy currents are generated in the metal bed as the metal bed moves relative to said first means. Said first means are conveniently adapted in such a manner that the generated magnetic field is directed towards, preferably substantially perpendicular to, the metal bed. In other words, the central lines of flux are preferably substantially perpendicular to the metal bed. Said first means may comprise one or more permanent magnets, one or more electromagnets with an iron core and/or a powder core, or a combination of permanent magnet(s) and electromagnet(s). A permanent magnet is preferably arranged so that the axis which is defined by the north pole and the south pole is substantially perpendicular to the metal bed. In the case of an electromagnet, the axis of the coil is preferably directed perpendicular to the metal bed.
The device further comprises second means which interact with the eddy currents and are adapted in such a manner that they are affected by a force which is related to the eddy currents. Preferably, the main direction of the generated magnetic field forms a substantially right angle to the direction of said force which affects said second means. Said second means comprise, for example, a magnet which is arranged on a pendulum or is included in a pendulum, magnets which are arranged on a wheel or are included in a wheel, or a fixedly attached element, in which stress arises under the action of the force.
The device also comprises third means which are adapted to detect the action of the force on said second means, the detected force being a function of the desired parameter of the metal bed. Said third means comprise, for example, Hall sensors, double-pulse lasers, strain gauges, pressure gauges, hydraulic or pneumatic means, magneto-resistive pulse sensors, optical pulse counters or ultrasonic devices.
Said first, second and third means are preferably all enclosed in a casing that belongs to the device, which, when measuring, is placed in a contactless manner on one side of the metal bed. The term xe2x80x9cone sidexe2x80x9d naturally comprises that the measuring is carried out from above, below, diagonally from the metal bed, or either side thereof, etc. In a Cartesian system of coordinates, the device preferably has one single main direction towards the metal bed.
According to a preferred embodiment of the present invention, said second means comprise a body which is adapted to be affected by a force so that bending or angling, preferably a pendulum swing, of said body is achieved. Thanks to the design of the device, which means that measuring is carried out from only one side, it is possible to obtain a great swing caused by said action of the force by using an elongated body.
According to another preferred embodiment of the invention, said second means comprise a body which is fixedly attached to or suspended from the device. Said third means are adapted to detect mechanical stress in the body which is caused by the force, the means being adapted to measure the stress which is a function of the induced force. Said third means can be strain gauges arranged on the body, the resistance of the strain gauges being changed as a result of the deflection of (stress on) the body. By arranging, for instance, two or more gauges round the body, also the direction of motion can be determined, apart from the speed of the metal bed. Naturally, other types of gauges may also be used, such as pressure gauges, piezoelectric gauges, hydraulic or pneumatic means.
According to yet another preferred embodiment of the invention, said second means comprise a body which is movably attached to or suspended from the device in such a manner that it moves under the action of the induced force. Such a movably arranged body may, for instance, be a suspended pendulum that comprises a magnet, said third means being adapted to measure the pendulum swing which is a function of the induced force. In order to minimise the resistance to the pendulum motion, the swing arm of the pendulum can be suspended from a point, such as from a tip. The force which is induced by the eddy currents strives to make the swing arm follow the direction of motion of the metal bed, which makes it possible to measure, for instance, both the speed and the direction. Another alternative is to suspend the swing arm cardanically. Yet another variant is to suspend the swing arm from an edge, which thus limits the indication of direction to one plane. Preferably, such a suspension is therefore turnable in order to make it possible to find a maximum swing and, thus, both the speed and the direction of motion.
In another embodiment according to the invention, said second means are directly or indirectly adapted to flow in a medium with higher density than air, for instance, a low viscosity fluid. The advantage of this embodiment is that the weight of said second means is smaller in such a medium, which results in the inertia of motion also being smaller. When using such a medium, a greater swing or deflection is thus obtained as regards the same action of the force than in the case without a medium, the swing or deflection also being more easily detected by said third means. Said second means may, for instance, comprise a float which encloses a magnet.
According to yet another embodiment, the body may be supplied with magnets, which are actuatable by a force, and rotatably arranged so that it rotates as a result of the action on the magnets as a consequence of the induced currents, said third means, for instance pulse counters, being adapted to measure the speed of rotation which is a function of the induced force. The rotatably arranged body may, for example, have the shape of a wheel with a wheel hub which is attached to a suspended link arm. The wheel is preferably arranged to rotate in planes that are perpendicular to the plane of the metal bed. In order to find the maximum speed of rotation (and thus both the speed and the direction of the metal bed), the wheel is preferably turnably and self-adjustingly suspended. Also other embodiments of rotatable bodies are conceivable, more or less symmetric, for instance, in the form of a sphere or a polygon.
Said first and second means can be arranged separately from each other or be connected to each other. They may also be part of a common unit, such as a permanent magnet in the form of a suspended pendulum. In this case, the pendulum, which is arranged for measuring from one side of the metal bed, generates a magnetic field which induces eddy currents, and moreover is affected by the force induced by the eddy currents. The advantage of constructing said first and second means connected to each other or in a common unit is that the device will be less bulky and easier to handle. It should be noted that the term xe2x80x9ca common unitxe2x80x9d also comprises the variant where said first and second means are one and the same means.
It has turned out to be advantageous to use Hall sensors as said third means when said second means comprise a movably attached body. A Hall sensor is essentially a conductor or a semi-conductor in which an electric current flows. When the sensor is subjected to a magnetic field that is perpendicular to the direction of current, a difference of potential appears across the sensor in a direction which is perpendicular to both the direction of current and the magnetic field. This so-called Hall voltage is directly proportional to the magnetic flux density. When the movably attached body thus changes position, for example, a pendulum comprising a magnet (i.e. the position of the magnet changes), then also the Hall voltage across the sensor changes, whereby the swing and, thus, the desired parameter can be measured.
It has also turned out to be advantageous to use one or more lasers as said third means. An advantage of using lasers is that the detection of the action of the force on said second means takes place in a contactless manner. Another advantage is that they can be used in connection with both a fixedly attached body and a movably attached body. For instance, holographic interterometry with a double-pulse laser may be used as light source. This means that said second means are subjected to double exposure at different instants of time. Between the exposures said second means undergo a change, such as a pendulum swing or bending. When the image of the double-exposed means has been reconstructed and the holographic interferogram is examined, a pattern of interference fringes will be visible. From this pattern, the change of the means between the two exposures can be determined. The reading of the pattern can, for instance, be made by a CCD camera.
The device according to the present invention has a limited, space-saving design and can be used in various technical fields. Its use in the metallurgical industry is particularly advantageous, which may comprise fields of application such as measuring speed and/or direction of the motion of a metal surface in a chill mould during casting. The non-circumferential device according to the invention can also be used for measuring the motion of a metal surface in various process containers, for example, the tundish in the manufacture of steel and metal, metal that flows in a duct, pipe or the like. The device is then placed by the side of the metal bed and, because of this assymmetrical measuring method, it can easily be re-arranged. It is also possible to measure on a medium having electric characteristics, for example, slurry of iron ore, a suspension of a metal, the motion of a metal in the surface of another material or the motion of a solid material which is exposed to processing, such as a band, a rod or the like.
A measuring signal which is obtained according to the invention can be used for systems that are utilised for controlling units which affect the speed of the metal bed, such as control of: magnetic brakes in a chill mould in casting, an agitator in casting, the speed of belts in process lines, rotating shafts, flow rate in casting or the reference level in a chill mould in casting to optimise the metal flow in the chill mould.
An advantage of the device according to the present invention is its xe2x80x9cbuilt-inxe2x80x9d vibration absorption. When vibrations, if any, appear, which in prior-art technique constituted a great disturbance, they will generate a counterforce in the device, the counterforce neutralising the vibrations, whereby the device will be self-absorbing.
Another advantage of the invention is that the measuring takes place in a contactless manner. Thus, the metal bed is not affected by the device, which is non-enclosing, that is, placed on one side of the metal bed, in any other way than by the magnetic field which is generated for the purpose of measuring and the resulting counterforce in the metal bed. The fact that the device is placed only on one side of the metal bed gives a great possibility of choosing the position in the longitudinal direction and/or vertical direction in which the measuring is to be carried out. It is also very easy to move or remove the device entirely without risking that it interferes with the metal bed. The device is, for example, particularly suitable for measuring in a chill mould since there is a possibility of getting very close to the metal.
If a constant magnetic field is generated by the device according to the invention, the detected force varies with the distance of the device to the surface. It may thus be advantageous to control the distance to the surface to be able to measure correct values of the speed and the direction of motion of the metal bed. This means that vice versa, when the motion of the metal bed is known, the distance can be determined instead.
According to an advantageous embodiment of the invention, the device which is separated from the metal bed comprises means for measuring both the motion of the metal bed and the distance thereto.
The invention has so far been described in connection with a moving metal bed. Those skilled in the art realise that, of course, it can also be used on a non-moving metal bed if one wishes to measure on the moving device instead, when, for instance, scanning the device above or beside the metal bed. The inventive device can thus be used in connection with a relative motion between the metal bed and the device.